Conventionally, LiCoO2 is mainly used as a positive active material for a nonaqueous electrolyte secondary battery. However, a discharge capacity of the LiCoO2 has been about 120 to 130 mAh/g.
A solid solution of LiCoO2 and another compound is known as a material of a positive active material for a nonaqueous electrolyte secondary battery. Li[Co1−2xNixMnx]O2 (0<×≦½), which has a crystal structure of an α-NaFeO2 type and is a solid solution of three components, LiCoO2, LiNiO2 and LiMnO2, is reported in 2001. LiNi1/2Mn1/2O2 or LiCo1/3Ni1/3Mn1/3O2 as an example of the solid solution has a discharge capacity of 150 to 180 mAh/g and is also excellent in charge-discharge cycle performance.
For the above-mentioned so-called “LiMeO2 type” active material, the so-called “lithium excess type” active material, in which a compositional ratio Li/Me of lithium (Li) to a ratio of a transition metal (Me) is larger than 1 and for example Li/Me is 1.25 to 1.6, is known. A compositional formula of such a material can be denoted by Li1+αMe1−αO2 (α>0). Here, when the compositional ratio Li/Me of lithium (Li) to a ratio of a transition metal (Me) is denoted by β, since β=(1+α)/(1−α), α=0.2 if Li/Me is 1.5.
In Patent Document 1, an active material, which is a kind of such an active material and can be represented as a solid solution of three components of Li[Li1/3Mn2/3]O2, LiNi1/2Mn1/2O2 and LiCoO2, is described. Further, as a method for manufacturing a battery using the above-mentioned active material, it is described that by providing a production process in which charge at least reaching a region, occurring within a positive electrode potential range of more than 4.3 V (vs. Li/Li+) and 4.8 V (vs. Li/Li+) or less, where a potential change is relatively flat is performed, it is possible to manufacture a battery which can achieve a discharge capacity of 177 mAh/g or more even when employing a charge method in which a maximum upper limit potential of a positive electrode at the time of charging is 4.3 (vs. Li/Li+) or less.
The invention of “a layered rock salt-type lithium transition metal composite oxide represented by Li1+XM1−XO2 (M is at least one transition metal selected from nickel, manganese, cobalt, iron, copper, zinc, chromium, titanium and zirconium; and 0≦×≦0.15), wherein the content of an acidic root is 1500 ppm or less in terms of a total amount, the content of an alkali metal is 2000 ppm or less in terms of a total amount, and the intensity ratio (I(003)/I(004)) of peaks of (003) and (104) lines in X-ray diffraction, which are indexed a hexagonal crystal, is at least 1.4” is known (see Patent Document 2). Further, Patent Document 2 describes a method for production of the lithium transition metal composite oxide (“lithium-excess-type” positive active material), wherein a compound of a transition metal element containing Co, Ni and Mn is coprecipitated to produce a coprecipitation precursor of a transition metal carbonate, the precursor is mixed with a lithium compound, and the mixture is fired at 800 to 900° C., and it is shown that by using the positive active material produced by the above-mentioned method, a nonaqueous electrolyte battery having a high discharge capacity and an excellent rate characteristic is obtained.